1. Field of the Invention
The present invention relates to a radiation imaging apparatus and its control method.
2. Description of the Related Art
Recently, there is known a radiation imaging apparatus using flat panel type sensor including a sensor array having a two-dimensional array of sensors (to be referred to as pixels hereinafter) each constituted by a conversion element for converting radiation into a signal charge (electrical signal) and a switch element such as a TFT for transferring the electrical signal to the outside. An amorphous silicon or polysilicon film formed on a glass substrate is used for a conversion element. In general, such a radiation imaging apparatus transfers the signal charge converted by the conversion elements to a reading apparatus by performing matrix driving using switch elements such as TFTs, thereby performing reading operation.
Upon application of radiation, each conversion element on the sensor array directly or indirectly generates a signal. In a sensor based on a system that directly generates signals, the conversion element of each pixel detects visible light converted by the phosphor instead of directly detecting radiation. In either a sensor based on the direct system or a sensor based on the indirect system, each pixel undesirably generates some signals even without application of radiation. Such a signal will be referred to as a dark current. Dark currents have different characteristics on the respective pixels on the array, and change with changes in the temperature of the sensor or over time.
Each pixel generates a dark current upon application of radiation in the same manner as when no radiation is applied. It is therefore possible to remove the influence of a dark current on an image by calculating the difference between a signal from each pixel with application of radiation and a signal from each pixel irradiated without application of radiation (Japanese Patent Laid-Open No. 2002-369084 (to be referred to as literature 1). That is, this technique separately acquires an image (to be referred to as a radiation image hereinafter) obtained by scanning the sensor array with application of radiation and an image (to be referred to as a dark image hereinafter) obtained by scanning the sensor array without application of radiation. The technique then obtains an image of the object by performing subtraction processing between the corresponding pixels of these images. Note that in order to prevent the occurrence of removal residues due to changes in dark current characteristic itself as described above, it is preferable to acquire a radiation image and a dark image in temporal vicinity to each other.
An imaging procedure in a general radiation imaging apparatus will be described with reference to FIGS. 1 and 2. When the user inputs an imaging trigger to the apparatus by pressing a hand switch to perform imaging, the apparatus performs initialization operation (S1) for the sensor array first. In this case, the apparatus sweeps the dark currents accumulated in the sensor before imaging, and makes adjustment to allow the sensor to properly perform photoelectric conversion. Sweeping of dark currents in initialization operation is similar to reading of images in terms of performing scanning operation of sequentially turning on the TFTs on the respective rows on the sensor array. However, this operation does not perform A/D conversion. In this case, therefore, no image data is generated.
Upon completion of initialization operation, the apparatus turns off all the TFTs on the sensor array to make the respective pixels independently ready for photoelectric conversion. In this case, this state is called an accumulation state (S2). When the sensor array is set in an accumulation state, the apparatus irradiates the object with radiation (S7). This makes the respective pixels on the sensory array convert the gradation information of radiation transmitted through an object into charge. This charge is accumulated in each pixel until the subsequent reading/scanning. At this time, each pixel has generated the dark current described above independently of radiation/charge conversion. As a consequence, a sum of the image and the dark current is accumulated in each pixel.
The end of application of radiation is determined based on various factors. Simply, the apparatus finishes the application of radiation when an irradiation time set in advance has elapsed. A more preferable system is designed to make a radiation measuring apparatus called a phototimer stop the application of radiation when the total dose of radiation which has reached the sensor reaches a given value. In any system, when the user expresses his/her intention to stop the application of radiation (for example, releases the exposure switch), the apparatus accepts the intention with the top priority. As described above, although it is not possible to determine when to finish the application of radiation, the apparatus finishes the application of radiation when the above conditions are satisfied.
Upon completion of application of radiation, the apparatus immediately reads signals (charge stored in S2) from the sensor array (S3). In reading operation, the apparatus turns on the TFTs on the respective rows on the sensor array to sample and hold charge signals transferred to the respective column signal lines and perform A/D conversion, thereby obtaining digital data corresponding to the pixels on the respective rows. In addition, sequentially scanning the rows of the TFTs which are turned on will obtain digital data from the overall two-dimensional sensor array.
In this case, the image data obtained by reading operation after the application of radiation, that is, a radiation image 10 shown in FIG. 2, is a sum of the halftone information of radiation and dark currents from the respective pixels of the array, as described above. Note that reading signals from the sensor array immediately after the application of radiation is effective in reducing the proportion of dark currents in the image and reducing residues in the subtraction processing to be described later. This operation also has an effect of shortening the delay time until the image is presented to the user.
Although the radiation image is acquired in the steps so far, the process enters the step of acquiring a dark image to remove dark current components from the radiation image. The apparatus starts dark image acquisition by performing initialization operation (S4) again immediately after reading the radiation image. Upon performing initialization operation again, the sensor array is set in an accumulation state (S5) again. The purpose of this accumulation state is to acquire a dark image from the sensor array. Therefore, the apparatus applies no radiation. The apparatus controls the duration of the accumulation state in dark image capturing so as to make it equal to “duration of accumulation state (S2) in radiation image capturing”. Note that in radiation image capturing, the duration of an accumulation state is determined on site but is not known in advance, whereas the accumulation time of dark image capturing will have been determined at the start of an accumulation state.
When a predetermined accumulation time has elapsed, the apparatus reads signals from the sensor array (S6). The reading method to be used is the same as that used to read the radiation image. The image obtained here is called a dark image 11 (FIG. 2). The apparatus has acquired the radiation image 10 and the dark image 11 in the steps so far. As described above, the dark current components superimposed on the radiation image 10 are almost identical to the dark image 11. To obtain a final captured image 12, therefore, the dark image 11 is subtracted from the radiation image 10. The above imaging procedure is described in literature 1.
In the above case, the apparatus executes the dark image acquisition step immediately after the radiation image acquisition step in order to match the accumulation time for a radiation image with that for a dark image and set a minimum necessary accumulation time. Depending on the characteristics of a sensor array and correction based on calculation, it is not necessary to match accumulation times. In such a case, it is possible to use a method in which the dark image acquisition step is set at any position other than after the radiation image acquisition step.
For example, there has been proposed an imaging procedure performed in the following steps. First of all, the apparatus periodically and repeatedly acquires dark images in a waiting state, writes the acquired dark images in a memory, and updates the old dark images. Therefore, latest dark images always exist in the dark image memory. When the user inputs an imaging trigger to the apparatus by, for example, pressing a hand switch for the execution of imaging, the apparatus executes the radiation image acquisition step. When obtaining a captured image, the apparatus subtracts a dark image from a radiation image. At this time, the apparatus corrects the dark image based on calculation, as needed.
Some apparatus obtains two types of captured images by combining them. That is, this apparatus obtains a captured image for immediate display by using a dark image acquired in a waiting state, and obtains a high-quality captured image by using a dark image acquired after the acquisition of a radiation image. In some cases, in such apparatuses, the resolution of a captured image for immediate display differs from that of a high-quality captured image.
A sensor array for radiation imaging must have a physical size almost equal to that of an object. For example, a sensor array designed for imaging the human body has a size of about 40 cm×40 cm. When a magnetic field is externally applied to an array wiring of this size, the array wiring itself operates as a sensitive magnetic field sensor.
A typical source which emits a variable magnetic field to an environment is an AC power source wiring for house facilities. When an AC power source current flows in the power source wiring, AC magnetic fields are generated around the wiring. The closer to the power source wiring, the larger the magnetic field generated in a space. In addition, the larger the power consumption of a device which receives power from the wiring, the larger the magnetic field generated. As a consequence, when a power source wiring which carries a large amount of power is placed near an imaging apparatus, a change in magnetic field crossing the sensor array during the reading operation of the array may be superimposed on an image, resulting in an artifact. This operation will be described below with reference to FIGS. 3 and 4.
As described above, when obtaining a captured image, the apparatus acquires a radiation image and a dark image and subtracts them from each other. The apparatus acquires each image by performing initialization, accumulation, and reading. Of these operations, initialization and accumulation are done without amplification and A/D conversion, and hence a magnetic field has no influence on an image. In contrast to this, in reading operation, the apparatus turns on the TFTs on the respective rows on the sensor array to sample and hold charge signals transferred to the respective column signal lines. At this time, when a magnetic field crossing the sensor array varies, electromotive forces are generated in signal lines, resulting in differences between sampled and held values. This phenomenon continuously appears while the apparatus sequentially scans the respective rows. As a result, changes in magnetic field during scanning appear as a fringe pattern on an image.
Such fringe patterns are superimposed on both a radiation image 20 and a dark image 21, as shown in FIG. 4. If the fringe patterns on the two images exist in the same phase, existing subtraction processing makes them cancel out each other, resulting in no fringe pattern on the captured image. If, however, reading operations start out of phase in a fluctuation cycle of an environmental magnetic field like the reading start timings indicated by S3 and S6 in FIG. 3, the fringe patterns on the radiation image and dark image shift from each other. If fringe patterns exist in opposite phases, subtraction processing will enhance them. As a consequence, an enhanced fringe pattern appears as an artifact on the captured image. In addition, a fringe pattern as a residue is left between the two images in accordance with the phase differences. As a consequence, correcting the radiation image 20 by using the dark image 21 will obtain a captured image 22.
In general radiation imaging apparatuses, what kinds of phase differences fringe patterns superimposed on a radiation image and a dark image exhibit are completely accidental. This is because, since an accumulation time for capturing a radiation image is determined during imaging operation, it is not possible to determine in advance the time difference between reading operation for a radiation image and reading operation for a dark image.